To investigate the role of matrix metalloproteinases (MMPs) in the mobilization of peripheral blood stem cells stimulated by granulocyte colony-stimulating factor (G-CSF), we analyzed MMP serum levels in 11 healthy donors and 9 patients who had hematological malignancies or germ cell tumors. A dose of 5-10 microg/kg per day of G-CSF (lenograstim) was administered for 4-8 days to each subject. The serum levels of MMP-2, and MMP-9; interleukin-3, -6, -8, and -10; stem cell factor; interferon-gamma; and tumor necrosis factor-alpha were measured both before and during G-CSF administration. MMP-9 was found to be increased in both the cancer patients and the healthy donor group. In contrast, the levels of each of the other factors tested were unchanged. No significant positive correlation was observed between the MMP-9 levels and the number of CD34+ cells. Hence, we found no significant role for MMPs during the mobilization of peripheral blood stem cells stimulated by G-CSF.